Certain industrial processes, such as heating a load in a furnace, rely on heat produced by the combustion of fuel and oxidant. Fuels include oil, natural gas, pulverized coal, and biomass. Oxidants include atmospheric air, vitiated air, oxygen, or air enriched with oxygen. Combustion of the fuel and oxidant causes NOx to result from the combination of oxygen and nitrogen.
An indurating furnace is a particular type of furnace that is known to produce high levels of NOx. Large quantities of pelletized material, such as pellets of iron ore, are advanced through an indurating process in which they are dried, heated to an elevated temperature, and then cooled. The elevated temperature induces an oxidizing reaction that hardens the material. When cooled, the indurated pellets are better able to withstand subsequent handling in storage and transportation.
The indurating furnace has sequential stations for the drying, heating, and cooling steps. In a straight grate furnace, a moving grate conveys the pelletized material into the furnace, through the sequential stations, and outward from the furnace. Air shafts known as downcorners deliver downdrafts of preheated air to the heating stations. Burners inject fuel and combustion air into the downdrafts, and the resulting combustion provides heat for the oxidation reaction that hardens the pelletized material.
Another type of indurating furnace is known as a grate-kiln furnace. It differs from a straight grate furnace by using a moving grate only for the drying and preheating steps. When those steps are completed, the pelletized material is transferred from the grate into a rotary kiln. A burner is fired into the rotary kiln to provide heat as needed to harden the pelletized material
In some cases, the burner for a rotary kiln is fueled with natural gas or oil. In other cases, the burner uses solid fuel such as pulverized coal or biomass. The solid fuel is delivered to the burner in a stream of conveyance air. Additional air may be delivered to the burner for cooling. Solid fuel may also be mixed with the pelletized material. A hood structure provides the rotary kiln with process air that is separate from the conveyance/cooling air at the burner. The process air includes combustion air needed for combustion of the fuel, and also includes air needed for the oxidation reaction in the pelletized material.